Jack-up platforms typically comprise a hull and at least three longitudinally movable support legs. The support legs are individually movable relatively to the hull, i.e. can be lifted or lowered, using at least one driving mechanism. Usually, each leg has at least one separate driving mechanism on its own.
The lower ends of the support legs have to be put on a fixed ground for preparing the platform for service. For this purpose, the support legs are lowered until they touch the ground. Then the hull can be jacked to any arbitrary position above the ground by correspondingly driving the support legs which results in a movement of the hull. The support legs can be arranged in parallel or can be slant to improve stability of the jacked-up platform. The ground may have an inclined and/or uneven profile. In this case, the support legs are driven to different positions to keep balance of the hull.
For off-shore jack-up platforms, typically the hull is designed to be floatable in the maximally lifted state of the support legs. Thus, such a platform can be easily transported to its service location, e.g., by dragging it along the water surface using tugboats. When the platform reaches its service position, the support legs are driven down through the water until each of them touches the seabed. The hull can then be jacked up above the water level to increase the load onto the support legs for a stable standing of the platform. These platforms are usually applicable in waters of a depth of up to 150 m, but not in the deep sea.
Jack-up platforms of this kind are used, for example, in off-shore operations of the oil and gas industry for exploring or exploiting subsea gas and oil fields. In other words, they can be used as mobile gas or oil rigs. Other applications for off-shore jack-up platforms are, for example, maintenance works on subsea pipelines or other subsea lines as well as bed works in rivers or port basins.
An advantageous driving mechanism for jack-up platforms has been disclosed in WO 2005/103301 A1. There, permanently excited electric motors (also called “permanent magnet motors”) have been proposed for moving the support legs and for holding the hull in a predetermined position above the ground, in contrast to induction motors used in prior art. This way, no mechanical brakes are needed for temporarily holding the platform, because the hull can be kept in position solely by the high-efficiency permanent magnet motors. Besides, the permanent magnet motors enable a movement of the support legs with infinitely variable speed, thus permitting smooth operations with high torque, in contrast to typical prior art with two-speed operation, high slip. However, no efficient way of controlling the driving mechanism has been disclosed so far.